In a conventional charging system for a vehicle battery, a lamp is provided to indicate that the generated voltage is above a given value so that the battery is properly charged. In order to energize the lamp, a relay responsive to the voltage from an auxiliary rectifier is employed, where the output voltage of the auxiliary rectifier is used to energize the field coil of the alternator. The field coil is also supplied with a current from the battery via a field resistor so that initial energization of the field coil is effected by the current from the battery. The relay has a normally closed contacts, and thus the lamp is energized when the generated voltage represented by the output voltage of the auxiliary rectifier is below a predetermined voltage. When the generated voltage exceeds the predetermined voltage, the relay is energized to deenergize the lamp, indicating that the alternator is generating a sufficient voltage for charging the battery.
However, the conventional charging system suffers from the following disadvantages or drawbacks. Namely, at the beginning of power generation, a very low voltage is generated and a current is supplied to the winding of the relay. As a result, the field coil of the alternator cannot receive a sufficient amount of current from the auxiliary rectifier, deteriorating the rising characteristic of the alternator. Although it is possible to make the resistance of the above-mentioned field resistor supplying an initial exciting current from the battery to the field coil large to improve the rising characteristic, the rated power of the resistor has to be increased because of the increase of the current passing therethrough. Accordingly, a bulky resistor has to be built in an assembly of the alternator.
The conventional relay used for controlling the energization of the charge-indicating lamp is energized when the voltage applied thereto exceeds a first given voltage, i.e. a minimum operating voltage, and is deenergized when the voltage lowers below a second given voltage, i.e. a return voltage, which is lower than the first given voltage. Because of the difference between the minimum operating voltage and the return voltage, if the voltage from the auxiliary rectifier is below the minimum operating voltage but is above the return voltage during energization of the relay, the relay is continuously being energized to cause the charge-indicating lamp to indicate that the generated voltage is above a predetermined voltage which is sufficient for charging the battery until the voltage from the auxiliary rectifier lowers below the return voltage. This means that the charge-indicating lamp does not correctly represent the charging and noncharging states of the system. Therefore, it is desired that the return voltage is as close to the minimum operating voltage as possible. In order to reduce the difference between the minimum operating voltage and return voltage the number of relay windings has to be increased, and this will result in the increase in size of the relay.
Moreover, the above-mentioned minimum operating voltage of the relay is not necessarily uniform throughout a number of relays having the same rated voltage. In other words, there are variations in minimum operating voltage throughout a number of relays, and therefore, some relays having desired minimum voltage have to be selected so that the relay will satisfactorily operate in accordance with the voltage applied thereto.